Conventionally, optical disks such as optical disks, magneto-optical disks, etc., for recording/reproducing information using a laser are known. In recent years, in order to realize a higher recording capacity, the techniques for increasing recording density of the optical disks have been developed, and with an increase in recording density, the recording pit becomes smaller in size.
In an optical pickup device for reproducing information recorded on an optical disk at high density, in order to read out the data recorded in a fine region, it is required to focus a light beam onto such fine region on the optical disk. Namely, by reducing the spot size, it becomes possible to record a greater amount of information.
The spot size is proportional to a wavelength λ of the light source, and is inversely proportional to the numerical aperture NA of an objective lens. Therefore, in order to reduce the size of a light spot, it is required to either reduce the wavelength λ to be used, or increase the numeral aperture NA of the objective lens.
For example, for CDs (Compact Discs), wavelength λ=780 nm; numerical aperture NA=0.45, and a recording capacity=650 MB. For DVDs, wavelength λ=650 nm; numerical aperture NA=0.60; and a recording capacity=4.7 GB. Therefore, the DVDs (Digital Versatile Discs) offer smaller light spot by reducing the waveform λ and increasing the numeral aperture NA. As a result, the DVDs have high recording capacity of around 7 times as high as that of the CDs, and thus a greater amount of information can be stored therein as compared to the CDs.
By the way, when performing recording/reproducing operation with respect to an optical disk, surface fluctuations of the optical disk are likely to occur. Here, to maintain the data recording surface of the optical disk within the focal depth of the objective lens, a focus control is performed to maintain an interval between the optical disk and the objective lens constant. For this focus control, conventionally, the Foucault method, the astigmatic method, etc., are known.
In the Foucault method, a knife edge is provided in an optical path of a light reflected from the optical disk. At the knife edge, a half of the reflected light from the optical disk is shielded. Two-split light receiving elements are provided at a focal point of the objective lens for use in detecting a reflected light when the objective lens is at focus with respect to the optical disk. When the objective lens is at a correct focus position, the reflected light to focus the two split light receiving elements will be balanced (impinges at the midway between the two split light receiving elements) so that the difference of the outputs of these two light receiving elements will be zero. On the other hand, when the objective lens is out-of-focus position due to the up-and-down movements of the optical disk, the reflected light impinges at position shifted to the left or to the right from the midway, resulting in a difference between a signal obtained from the left receiving element and a signal obtained from the right receiving element. Then, by detecting this difference in signal, a focus error signal is obtained.
In the astigmatic method, optical components including a cylindrical lens, an oblique glass plate, etc., are provided on the optical path of the light reflected from the optical disk. As a result, the reflected light becomes astigmatic, and two caustic lines appear so as to cross at right angle. In response, four-split detectors are provided in a minimum circle of confusion in around the midway of the two caustic lines. The direction of the two pairs of the light receiving elements in the four-split detectors is set to the direction of the two caustic lines. When the optical disk is at a correct focus position, the respective total amounts of light received by each pair of the two light receiving elements which face one another are equal. On the other hand, when the optical disk is at out-of-focus position as being moved up and down, a total amount of light received by one pair will be different from that received by the other pair. Therefore, a focus error signal is obtained by detecting a difference in signal obtained from the respective pairs of the light receiving elements.
In the described focus control, the reflected light from the optical disk is converged onto the light receiving element by the focus error signal detection-use condenser lens, and the amount of light received by the light receiving element at the focus spot is processed with signals. As a result, a focus error signal is detected. In the focus control, in order to ensure that the focus error signal to be detected has a predetermined signal level, the objective lens is shifted by controlling the driving of an actuator having mounted thereon the objective lens, and then a focus servo loop is formed. In this way, an interval between the data recording surface of the optical disk and the objective lens can be maintained constant, and a laser beam can be kept at a focal point.
When performing a focus control, a focus servo loop is formed based on a focus error signal so that the information recording surface of the optical disk is positioned at an optimal image surface of the objective lens. The range where the focus servo loop satisfying the above condition can be formed almost corresponds to a range just before the light spot starts being displaced to the outside of the light receiving surface of the light receiving element for use in the focus error signal detection use optical system. Therefore, the range where the focus servo loop satisfying the above range can be formed depends on the relationship between the objective lens and the longitudinal magnification of the focus error signal detection use condenser lens, as well as the area of the light receiving surface of the light receiving element.
In recent years, with reduction in size and weight of the optical disk devices such as CDs, a light-weight, miniaturized optical pickup device is also demanded. Such light-weight, miniaturized optical pickup device can be realized, for example, by reducing the effective diameter of the objective lens. In this case, a focal distance of the objective lens is shortened. On the other hand, in order to increase the recording capacity of the optical disk, it is required to reduce the spot size. To this end, if the numerical aperture of the objective lens is increased, the focal length of the objective lens will be still shortened.
When the focal length of the objective lens is reduced, however, from the relationship between the objective lens and the longitudinal magnification of the focus error signal detection use condenser lens, a range where the focus servo loop can be formed becomes smaller.
Therefore, when recording or reproducing information on and from the optical disk, if the optical disk or the optical pickup device has some impact, or dust particles adhere onto the recording surface of the optical disk, the oscillation wavelength of the semiconductor laser abruptly changes within the range of several nms. For example, in an event that the mode hopping phenomenon, i.e., the focal point of the optimal image surface of the objective lens is displaced, occurs, the objective lens may be displaced from the range where the focus servo loop can be formed. Further, in the optical pickup device, when the objective lens is once displaced from the range where the focus servo loop can be formed, a focus pulling-in operation is performed by fluctuating the actuator having mounted thereon the objective lens. In this way, the focus servo loop is formed again.
Here, for the objective lens having a shorter focal distance, in general, the distance (working distance) between the optical disk and the objective lens is short. Therefore, the objective lens may hit the optical disk. When some damage occurs between the objective lens and the optical disk, the optical pickup device may be damaged.
Here, in order to prevent the objective lens from being displaced from the range where the focus servo loop can be formed, this range for the formation of the focus servo loop may be increased. To this end, however, for example, when the area of the focus error signal receiving element is increased, the response rate of the light receiving element is reduced, thereby adversely affecting the high speed detection of the reproducing signal from the optical disk.
The range where the focus servo loop can be formed may be increased by reducing the focal length of the objective lens in consideration of the relationship between the objective lens and the longitudinal magnification of the focus error signal detection use condenser lens, a shift amount of a beam focal point of the condenser lens with respect the shift amount of the magneto-optical disk becomes smaller, resulting in a lower detection sensitivity of the focus error signal.